1. Technical Field
This invention relates to the art of robotically moving objects between precise stations, and more particularly to compliance devices to accommodate misalignment in position or misalignment between portions of the workpiece prior to carrying out such movement or transfer.
2. Description of the Prior Art
Automatic mechanical assembly of precision fitting parts requires a method to accurately locate the parts relative to each other so meeting or interfit takes place without interference or contact. One example is the insertion of electronic elements, having ceramic, plastic or dipped portions provided with fragile wire leads extending therefrom, into mating, close-fitting lead receptacles. Although the elements can be stationed in an accurate spatial position by planting the leads in a fixture, the ability to pick up such elements depends upon gripping directly the ceramic portion thereof, which portion is cast, dipped or molded, and does not have the same tolerance of shape and definition that the wire leads have. Therefore, there can be a significant variation or misalignment between the wire leads and the ceramic body of the elements inhibiting accurate transfer or movement of such elements. If the robotic gripping tool were to force the element to adapt to its own preprogrammed positioning, the forces applied to the element would cause breakage or distortion, particularly at the leads.
The prior art has used compliant devices, slave plugs, and vision sensors to solve related problems. Compliant devices require lead-in edges on the receiving opening to permit offcenter mating of the workpiece and receptacle; the compliant device responds to sizable side thrust of the misaligned workpiece to provide centering. The requirement for lead-in edges frustrates the design of the control unit and must be carried out at slow speeds to avoid damage to the parts during offcenter mating. But, most significantly, such technique cannot be used with fragile parts such as electronic components. Some compliance devices use flexible or deformable material, captured between two metallic plates. The flexible material will deform to provide compliance. Compliance devices of this type are severely limited in that they do not provide the amount of compliance needed in many applications require relatively large side loads and are subject to fatigue failure problems. Many compliance devices use springs or hydraulic mechanisms to allow for some degree of flexibility in an angular or lateral direction (see U.S. Pat. Nos. 4,098,001; 4,439,926; 4,179,783; and French No. 2,445,199). The difficulty associated with these structures is that each structure uses an elastic mechanism to connect the major parts of the mechanism. These elastic devices are in permanent contact with such members, thus requiring relatively large side loads to affect compliance. Additionally, each device uses a rigid bearing member, such as ball bearings, to reduce the friction of movement. These rigid members add to side loading even though they roll or slip. Bearings and other sliding materials are subject to wear, thus adding to side loading. Moreover, these spring or flexure-based systems acquire hysteresis losses over time.
Slave plugs necessitate extra strokes of the assembly due to the need to stroke the parts together using the slave plug for alignment and again when the actual workpiece is inserted into a receptacle. Due to the possible contact between the slave plug and the part, the speed of such assembly method must be undesirably slow to avoid damage.
Vision or laser sensors often use search routines that comprise trial and error to find the correct alignment; the insertion carriage travels to where it believes the opening resides according to a hunt routine. This is not only time consuming, expensive and complicated, but also is not reliable in precisely locating the opening for precise, noncontact alignment. Vision systems often require markings (such as fiducials) or other preparations on the workpiece so that the robot may properly identify the workpiece or the workpiece's location and orientation.
Robot wrists are known which incorporate bearings between surfaces to allow a male part to be readily introduced into a bore when the end of the part and/or the opening of the bore are chamfered; free movement of the end piece with respect to the robot arm provides self-centering of the end piece but at the expense of complexity. Such robots are described more fully in U.S. Pat. Nos. 4,337,579; 4,595,334; 4,661,037; and French patent No. 2,506,195. Magnets have been introduced to such compliant devices with little success.
Two Russian patents use arcuate bearing joints, in one case by hydraulic drive means and in the other by an electromagnetic drive to achieve flexibility (see Russian patents SU-872-250 and SK-872-256); these devices are not responsive to microforces received from the workpiece to be compliant. In French patent No. 1,504,027, a magnetic member formed by two radially magnetized concentric rings separated by an air gap and, on the other hand, a part to be centered, which is made from a plastic material and in which are embedded metal bars or balls. Glass balls may be substituted to reduce friction and facilitate centering. For practical reasons, this magnetic device cannot be used in known robot wrists. These devices also do not provide for fixing or locking the position of a held part after it is retrieved.
Another disclosure, U.S. Pat. 4,609,325, provides a robot wrist having a connector with two elements magnetized in the direction of their axes and which are interposed between the robot arm and the end piece; they are disposed to attract each other mutually while being separated from each other by magnetized balls. When the workpiece provides a lateral force on the end piece, the wrist complies via the freedom of movement in the magnetic balls, but the force threshold is somewhat high; in the absence of a lateral force, the balls center by dividing themselves homogeneously between the two magnetized faces. The difficulty with this disclosure is that it does not provide an ultralow friction bearing, limiting its usefulness, and there is no locking means to ensure fixed positioning relative to the axis of the part.
It is therefore an object of this invention to provide a method and apparatus for precisely transferring or locating fragile and close-fitting parts, there being variable alignment between the portion of the part gripped and the portion of the part received in a receptacle.